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Julia implementation of Pentti Kanerva's sparse distributed memory.
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Julia implementation of Pentti Kanerva's sparse distributed memory.

The essential content of this repository is an IJulia/Jupyter notebook with code to accompany the article "The Mind Wanders," published in July 2018 on The primary reference for all the underlying ideas is the following book: Kanerva, Pentti. 1988. Sparse Distributed Memory. Cambridge, Mass.: MIT Press.

The notebook file sdm.ipynb was written for Julia version 0.6.3. The file sdmj7.ipynb is updated to run under Julia 0.7 or 1.0.


Sparse distributed memory is all about computing with high-dimensional binary vectors—long sequences of 0s and 1s interpreted as the coordinates of the vertices of a hypercube in a high-dimensional space. In the examples considered here the length of the vectors is generally 1,000 bits, and so there are $2^{1000}$ possible vectors, or patterns.

The data structure I have chosen for representing these patterns is the Julia BitVector, an array of single bits, packed into 64-bit words. The standard Julia input and output routines interpret these bits as Boolean values (false and true), but I promise it's safe to think of them as 0s and 1s. A big advantage of using BitVectors is that primitive bitwise operations are executed in parallel on all the bits of a machine word. On a 64-bit processor, xor-ing two 1,000-bit vectors requires only 16 operations instead of 1,000. (As it happens, xor is the operation we're most concerned with.)

This code was written with the aim of exploring the ideas behind sparse distributed memory. It does not create a practical system for using sparse distributed memory. (I'm not sure such a system can exist without specialized hardware.)

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